1. Field of the Invention
The present invention relates generally to the manipulation of light in optical communication networks and fiber optic systems. Particularly, the present invention relates to elements disposed within the light stream of a fiber optic communications network, such as a DWDM network. More particularly, the present invention relates to anti-reflective structures for use in semiconductor lasers and amplifiers.
2. Related Art
Optical communication systems are a vital part of today""s communication networks. In a typical optical communication system, information-containing optical signals are transmitted along an optical fiber. Lasers are important components of optical communication systems for producing the information-containing signals. Information is modulated upon light signals emitted from the laser into the optical fiber. Initially, optical fibers were used to carry only a single data channel at one wavelength in one direction. Technology, such as wavelength division multiplexing (WDM), has increased the amount of information transferred over a single optical fiber communication system by multiplexing several signals on a single fiber. WDM allows the launching and retrieving of multiple data channels in and out of an optical fiber. Each data channel is transmitted in a unique wavelength region. The WDM wavelength regions are typically hundreds of nanometers apart.
An improvement on WDM is dense wavelength division multiplexing (DWDM), which allows channels of information to be propagated at different wavelength regions that can be spaced spectrally at a set wavelength or frequency distance apart from one another. For DWDM, information channels may propagate at industry standard spacings that may be on the order of one or two nanometers apart from one another. Typically, DWDM systems operate with fine wavelength separations from one laser to the next of about 0.8 to 1.6 nanometers (nm). Thus, for DWDM, lasers must operate at precisely determined wavelengths to avoid interference with other channels.
DWDM lasers and many communications lasers typically comprise a semiconductor lasing medium contained within an optical cavity. Such lasers are frequently referred to as diode lasers, semiconductor lasers, or laser diodes. A laser operates through stimulated emission, whereby light is emitted from excited atoms or molecules of the lasing medium. In operation of the laser, the lasing medium is disposed in an optical cavity of the laser that is situated between two reflectors. Light intensity is amplified as the photons oscillate back and forth between the reflectors, which stimulates further photon emission in the lasing medium. For this reason, the lasing medium is sometimes referred to as a gain medium. By making at least one of the reflectors partially transmissive, monochromatic radiation is emitted when the light passes through the reflector. Monochromatic radiation is typically radiation containing a narrow wavelength region of light. This wavelength region of light may appear as one visible color, for example, red, blue, green, or yellow, etc., or it may be invisible, for example, ultraviolet or infrared light. Infrared light is typical of DWDM optical networks.
Fabry-Perot diode lasers are a type of laser in which the cavity reflectors can be formed by the boundary surfaces of a semiconductor material itself. The boundary surfaces are typically referred to as front and rear facets, wherein the front facet is the port of primary light emission. Thus between these mirrored facets lies a section of semiconductor material that comprises a waveguided gain medium. The optical cavity is formed between the front and rear facets. Typically, the rear facet strongly reflects light resonating within the cavity. The front facet partly reflects the light resonating within the cavity and partly transmits the light at a laser wavelength region. The laser wavelength region is the wavelength region of light emitted from the laser; it is defined, at least in part, by the length of the cavity. The light reflected back from the front and rear facets sustains oscillation by interacting with excited atoms to drop electrons from the upper level to the ground state, thereby emitting photons. Various parameters associated with the optical cavity can be adjusted to alter the wavelength characteristics of the emitted light, such as varying the spacing between the front and rear facets. For example, the cavity may be so short as to only allow the laser to operate in xe2x80x98single modexe2x80x99 and thereby only emit light at a single wavelength position; the length of this cavity can be varied to xe2x80x9ctunexe2x80x9d this spectral position.
While Fabry-Perot lasers are inexpensive to produce, their output is not optimized for many applications. They typically operate in a multi-mode state wherein a broad spectral range is emitted. This undesirable characteristic arises from the energy reflection that occurs at the facet surfaces. The reflection is due to the refractive index of the semiconductor lasing medium being high relative to that of an adjacent medium such as air. The difference in refractive index produces a strong, innate reflection in the laser. This reflection, which defines one of the cavity mirrors and is responsible for the basic operation of the laser, is called Fresnel reflection. The lack of monochromacity of the energy output is undesirable for many optical networking applications. The plurality of radiation patterns, or longitudinal modes, that occur within the optical cavity of a Fabry-Perot laser are linked to this lack of monochromacity.
If the output of the laser is not controlled, or is xe2x80x9cfree running,xe2x80x9d multiple longitudinal modes will be seen in the output. In a typical free-running Fabry-Perot diode laser deployed in an access network employing the synchronous optical network (SONET) standard, the energy output (intensity) is often situated in the 1310 nm window. Typically, the longitudinal modes are spaced approximately 0.6 nm apart and are distributed over many nanometers, approximately 30. Additionally, the spectral output is subject to drift with temperature and other influences. Consequently, the output is unsuitable for dense wavelength division multiplexing, because of spectral overlap of adjacent DWDM signals. The output is also not well suited for high-speed fiber optic communications at wavelengths that are displaced from the optical fibers zero chromatic dispersion point, because of signal degradation due to chromatic dispersion.
By addressing the innate facet reflections and/or their influence on the longitudinal modes of a Fabry-Perot diode laser, attempts have been made to improve the quality of laser output. The reflections can be either redirected or suppressed so that the natural modes are eliminated or minimized. Free from these reflections, a number of techniques are available to stabilize the laser so that it emits light at a single, or at least tightly confined, wavelength position.
To overcome the above problems, semiconductor distributed feedback (DFB) lasers are frequently used in DWDM communications networks as an alternative to free running Fabry-Perot lasers. DFB lasers utilize wavelength-selective feedback to make certain modes in the optical cavity of the laser oscillate more strongly than others, thus causing the laser to output light in a tightly confined wavelength region. DFB lasers attempt to achieve precise wavelength control, thereby allowing lasers to be used with DWDM systems. DFB lasers typically include a semiconductor gain medium, as discussed above, but with an added feature that controls the output wavelength of the laser. The semiconductor gain medium of a DFB laser is imparted with a periodic (repeating) structure that limits the emitted light to a tightly defined wavelength region. The periodic structure may be a corrugated structure or an undulation in refractive index. The structure is designed so that light of a particular wavelength will be constructively reflected back and forth to achieve lasing. Lasing approaches single mode operation, with the result of preventing other unwanted longitudinal modes from lasing. However, DFB lasers are not easy to manufacture because the techniques required to produce the periodic structure are cumbersome and are not readily reproducible to generate proper parameters. Additionally, DFB lasers are temperature sensitive because the output wavelength of DFB lasers varies as the temperature changes. They are also susceptible to performance variations due to the refractive index of the semiconductor medium varying with the intensity and propagation of output pulses of light. Consequently, DFB lasers have many shortcomings.
Another method for overcoming the problems of conventional laser devices, such as Fabry-Perot lasers, involves suppressing reflection produced by the facet. One suppression technique employs an antireflective coating formed on the facet. The coating is formed by depositing a stack of thin-film layers having high-low, alternating refractive indices to the facet. Fresnel reflections arising from the different refractive index interfaces between layers set up a pattern of constructive and destructive interference that minimizes the reflection from the facet. However, there are difficulties associated with designing and fabricating such a coating for practical application, as the performance tolerances are formidable. Another disadvantage arises from the inherent difficulty in attaining anti-reflection over a broad spectral band. Another disadvantage arises from the tight precision that is required for the thickness of the layers.
Another conventional approach for suppressing facet reflection involves angling one of the facets so that its internal reflections are not cast back towards the other facet. Consequently, a condition of back-and-forth reflections from front and rear facets is precluded so that longitudinal modes are suppressed. In addition to fabrication difficulties, this approach is inefficient. The optical axis of the emitted beam is bent by the angled facet with respect to the longitudinal axis of the semiconductor gain medium. This results in alignment problems and assembly complications when coupling light into various optical elements.
Other light amplification devices suffer from the same disadvantages discussed above. For example, a semiconductor optical amplifier (SOA) is constructed and operates somewhat similarly to the laser described above. SOAs have the capability of directly amplifying an optical signal without first converting it to an electrical signal. accordingly, SOAs are useful as repeaters, preamplifiers, and amplifiers in optical communication systems. Conventional SOAs are constructed somewhat as a modified laser diode. An optical gain region is formed between two facets, but the facets are typically covered with antireflection coatings so that the SOA operates as an amplifier rather than a laser. Alternatively, the facets may be angled with respect to the longitudinal axis of the gain medium and/or the flow of light within this medium so as to reduce the light reflected within the gain medium. Consequently, there is no resonant cavity. An SOA differs from a laser in that light does not reflect back and forth within the gain medium. Rather, an SOA receives an optical signal through one facet, amplifies the optical signal as it passes through the gain medium, and then transmits the amplified optical signal through the opposite facet. The optical signal passes through the SOA essentially one time during which it generates stimulated emissions that build up so as to amplify the signal. For this reason, SOAs are often referred to as a type of xe2x80x9ctraveling wave amplifier.xe2x80x9d SOAs suffer from the same disadvantages as conventional lasers. For example, light emitted through the facet of the SOA is deteriorated due to residual, unwanted reflection from the facet that is insufficiently suppressed by the antireflective coating or angled facet.
Accordingly, there is a need in the art for improved approaches to minimizing the reflections from facets of semiconductor lasers and other related semiconductor gain devices. Additionally, there is a need in the art for improved approaches to suppressing the unwanted longitudinal modes of semiconductor lasers and other related semiconductor gain devices. Specifically, there is a need in the art for a system and method that provides laser output while suppressing light reflected by the facet and while suppressing the innate longitudinal modes of the laser. Additionally, there is a need for a system and method that allows amplification of an optical signal while suppressing light reflected by the facets of the optical amplifier. Furthermore, there is a general need to suppress reflections from the optical surfaces of optical elements that are disposed in the light path of fiber optic communication networks, including DWDM networks.
The present invention can solve the problems of conventional laser devices by reducing or eliminating reflection from laser facets and by suppressing unwanted longitudinal modes produced in the optical cavity of the laser. Accordingly, the present invention can allow more precise wavelength control of the light output of a laser. Suppressing the unwanted longitudinal modes output by a laser can provide an output that approaches a single wavelength, or is at least tightly confined within a narrow wavelength spectrum, thereby allowing the laser to be used in a dense wavelength division multiplexing system.
In an exemplary semiconductor gain device according to the present invention, an optical cavity can be formed between two reflectors. A semiconductor gain medium can be disposed in the optical cavity wherein the semiconductor gain medium includes a front and rear facet, and a patterned structure can be disposed on one of the facets. When the semiconductor gain medium is excited to produce light emissions propagating through a facet of the device, the patterned structure can suppress the laser""s unwanted longitudinal modes of output, by suppressing reflection of the light from the facet. The patterned structure may be a xe2x80x9cmotheyexe2x80x9d pattern having a plurality of conical posts disposed on the surface of the facet. The conical posts may also have a pyramid structure, or the patterned structure may include lines, cones, holes, grids, or microlens arrays.
The present invention can also improve the operation of semiconductor optical amplifiers (SOAs). The patterned structure described above may be disposed on facets of an SOA to minimize emitted light reflected by the facet and to suppress any unwanted longitudinal modes output by the amplifier. Accordingly, the present invention can improve the performance of SOAs, thereby allowing them to be deployed in optical networks and integrated into Planar Lightguide Circuit (PLC) devices and systems.
The present invention may be combined with conventional devices to improve the operation of optical communications systems. By integrating together novel approaches to facet reflection with conventional laser control structures, for example, a MicroElectroMechanical Systems (MEMS) component, the present invention can provide laser control in communications lasers and related devices. A MEMS component can provide external tuning of the laser cavity to further refine the laser wavelength according to various usage parameters. As another example, a grating such as a fiber Bragg grating, or other wavelength-selective reflector, can be positioned into the path of light emitted from the front facet of a laser gain medium wherein the front facet is imparted with the novel antireflection structure of the present invention. In this exemplary configuration, the rear facet provides one cavity mirror and the grating provides the opposing, wavelength-selective mirror that effects a precise wavelength output. Additionally, the present invention may be integrated in PLC devices and systems to improve the optical interconnections between high and low refractive index materials in the optical transmission path.